Liquid metal current limiting device

ABSTRACT

An electrical current limiting device employing a liquid electrically conductive metal such as mercury pressurized in an enclosed container at a pressure slightly above its critical pressure. The pressure is confined to a narrow range where the specific conductivity of the mercury may vary by approximately six orders of magnitude for a change in the temperature of the mercury from near room temperature to its critical temperature as the I2R heating in the liquid is increased due to increase in electrical current. By operating the mercury current-limiting device at a pressure slightly above its critical pressure and a temperature equal to its critical temperature very effective current limiting will take place without the mercury changing state.

11 3,821,680 June 28, 11974 LIQUID METAL CURRENT-LlMITlNG DEVICE [75] Inventor: Howard C. Ludwig, Pittsburgh, Pa.

[ 73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Sept. 14, 1973 [21] Appl. No.: 397,565

[52] US. Cl 337/114, 337/21, 337/119,

337/158 [51] lint. Cl. H0111: 87/00 [58] Field of Search 337/21, 114, 116, 117,

3,699,489 10/1972 lmajyo 337/21 3,735,309 5/1973 Hurtle 3,747,040 7/1973 lnoue et al 337/21 X 3,753,190 8/1973 Tosliio lto ct a1. 337/21 Primary Examiner-A. T. Grimley Attorney, Agent, or Firm-M. .1. Moran 7 1 ABSTRACT An electrical current limiting device employing a liquid electrically conductive metal such as mercury pressurized in an enclosed container at a pressure slightly above its critical pressure. The pressure is confined to a narrow range where the specific conductivity of the mercury may vary by approximately six orders of magnitude for a change in the temperature of the mercury from near room temperature to its critical temperature as the 1 R heating in the liquid is in creased due to increase in electrical current. By operating the mercury current-limiting device at a pressure slightly above its critical pressure and a temperature equal to its critical temperature very effective current limiting will take place without the mercury changing state.

9 @laims, 2 Drawing Figures 1 mourn METAL CURRENT-LIMITING DEVlCE BACKGROUND OF THE INVENTION The subject matter of this invention relates generally to electrical current limiting devices and particularly to electrical current limiting devices of the nonexpandable ty'pe utilizing mercury or a similar electrically conducting liquid metal or alkali for providing a limitation to current.

The use of mercury or other alkali materials in elec trical current limiting devices is known where a heating effect causes the liquid to change to a gaseous high resistance state which thereby establishes an are between spaced terminals. The are provides current limitation and is usually extinguished at a current zero. Thereafter, the electrically conductive path including the mercury remains blocked until the gaseous material condenses to the liquid state. In these cases, the vaporization of the mercury which is critical in the current limiting process has the detrimental effect of relying on the production of an electrical arc for current limiting purposes. The electrical arc has the detrimental effect of causing deterioration of the contacts across which it is impressed or the chambers in which it is established. The result of this deterioration is to provide a finite limit for the number of times that the current limiting operation may take place before the current limiting device is rendered non-usable. Examples of United States patents utilizing the above-mentioned vaporization'arc producing phenomena are as follows: U.S. Pat. No. 3,117,203 issued to R. L. Hurtle on Jan. 7, 1964; U.S. Pat. No. 3,218,412, issued to R. T. Casey on Nov. 16, 1965; U.S. Pat. No. 3,259,867 issued to A. Latour on Jan. 5, 1966; U.S. Pat. No. 3,389,359, issued to L. P. Harris on June 18, 1968; U.S. Pat. No. 3,389,360, issued to J. J. Keenan on .lune 18, 1968; U.S. Pat. No. 3,430,017, issued to B. Storsand on Feb. 15, 1969; U.S. Pat. No. 3,452,172, issued to F. Kesselring on June 24, 1969; US. Pat. No. 3,454,833, issued to R. L. Hurtle on July 8, 1969, U.S. Pat. No. 3,488,761, issued to Toshio Ito et al. on Jan. 6, 1970; U.S. Pat. No. 3,501,730, issued to Ito et al. on Mar. 17, 1970; U.S. Pat. No. 3,559,138, issued to Itoh et al. on Jan. 26, 1971, U.S. Pat. No. 3,611,237, issued to Yamagata on Oct. 5, 1971 and U.S. Pat. No. 3,670,282 issued to Itoh et al. on June 13, 1972. Another example ofa current limiting switch is described in U.S. Pat. No. 3,617,807 issued to Kesselring on Nov. 2, 1971 in which a circuit interrupter is provided with a parallel connected resistor for current limiting purposes which'is cooled and maintained in the cooled state. In this device, a solid resistor is placed in parallel with the main electrical contacts and a constant flow of coolant supply is passed over the resistor for maintaining it at the previously described cooled temperature. One of the disadvantages of this current limiting device is the fact that constant energy must be supplied to the device to maintain the resistive element in a cooled state. In addition, the cur" rent limiting element is in parallel with the main circuit elements and does not act to carry the electrical current at all times. Another current limiting device is de scribed in U.S. Pat. No. 1,863,253, issued to H. L. Polin on June 14, 1932. This current limiting device employs an open bath of electrolytewhich is in the form of a pastry or gelatin mass comprising solids in liquid suspension where two electrodes are provided in the bath.

The heating of the bath by the current flowing therethrough has a current limiting effect as the temperature increases. However, this invention has the disadvantages of not being disposed in a closed container so that it may not be used at all attitudes. In addition, the electrolyte of this invention comprises a suspension of many materials rather than a relatively pure liquid elec' trically conducting material such as mercury. The invention has the additional disadvantage of requiring relatively large electrodes of positive and negative polarity and different composition. As an example, the positive electrode is defined as comprising mossy granite or the like and the negative electrode is defined as comprising electrometallic silicon or the like. A further disadvantage lies in the fact that no alternating current operation is defined for the electrochemical current interrupter. Another disdvantage lies in the fact that the relatively large bath of material is not conducive for use in a relatively thin or small diameter capillary tube where the effects of temperature and pressure can be best utilized for optimum sensitivity to change. An additional disadvantage lies in the fact that one of these suggested elements for the electrolyte is hydrochloric acid which has the property of giving off gaseous substances under the influence of high temperatures such as are found in an overload current condition. It would be advantageous to provide an electrical current limiting device for use over a wide range of currents and voltages which employs the use of a liquid metal electrically conductive material such as mercury confined to a small restriction where the effects of temperature and pressure may be best utilized for increase sensitivity. It would also be advantageous to provide the elec trically conductive metal in an apparatus which would not rely upon or cause electrical arcing or vaporization of the electrically conducting liquid material while providing a very efficient current limiting operation in response to the heat or overload current. The absence of an electrical arc in-a current limiting operation greatly extends the life of the current limiting device. For example, it is known that reusable electric current limit ing fuses has an operating life of approximately 40 to 50 current limiting duty cycles after which the effect of the electric arc struck in the vapor of the geated liquid metal so badly deteriorates or degenerates the contacts and chambers into which an arc is struck that the fuse must be replaced. It would be advantageous therefore to provide a fuse for remote use that could be exposed to a current limiting duty cycle many times without deteriorating or breaking down. It would also be advantageous to provide a fuse which could be utilized in underwater applications or deep space applications where servicing or replacement of the current limiting device is impossible or inconvenient. It would also be advantageous to provide a current limiting device which has enhanced current limiting properties such as the insertion of current limiting resistance in the order of six orders of magnitude a fault or overload electrical current is sensed.

SUMMARY OF THE INVENTION In accordance with the invention, an electrical current limiting device is provided which relies on the property of all liquid, electrically conducting materials of remaining in the liquid state while providing a drastic change in conductance if the material is maintained at a critical pressure and exposed to a critical temperature. An apparatus is provided for use with the liquid metal mercury which convenientlyhas a critical pressure and critical temperature which are relatively easy to attain and maintain in laboratory and commercial apparatus without the need for expending great sums of money and energy. Liquid mercury is provided in a capillary tube of an apparatus where the pressure of the mercury can be controlled at its critical pressure without the necessity of maintaining energy for keeping the mercury pressurized. Relatively reliable seals will keep the mercury pressurized withing a reasonable range once the proper pressure has been attained. The size of the tube and the properties of mercury are such that when the mercury is at room temperature, electrical conduction takes place. However, as an overload current appears the mercury is caused to heat without vaporizing to a temperature which is dependent upon the amount of overload current, which temperature is known as the critical temperature and at which point the liquid mercury rapidly changes conductivity by a factor of six orders of magnitude causing greatly enhanced current limiting properties to take place in the current limiting device.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention reference may be had to the preferred embodiment exemplary of the invention shown in the accompanying drawings, in which:

FIG. 1 shows a plot of the specific conductivity of mercury versus the pressure of mercury, where the critical temperature and pressure are displayed and where the operating range for the current limiting device is shown; and

FIG. 2 shows a side elevation partially cut away of an exemplary liquid state current limiting device utilizing the characteristics of mercury as a current limiting element as shown by FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIG. 1 in particular, a plot of the specific conductivity of mercury versus pressure with temperature as a parameter is shown. The information depicted in the curve of FIG. 1., arose from an extensive study of mercury concerning its conduction behavior by F. Hensel and E. U. Frank in the Berichte der Bunsengesellschuft, Bd. 70, Nr9/ 10, 1966. The study was associated with the conduction behavior of mercury near and above its critical values of temperature and pressure. The critical temperature is that temperature above which a gas cannot be liquified by pressure alone. The critical pressure is that pressure below which a substance may exist as a gas in equilibrium with a liquid. Mercury is a metal for which a critical pressure is tenable under present laboratory and commercial facilitation. According to the results of Franck and Hensel the critical pressure for mercury is 1,510 i- 25 bar and the critical temperature is 1490 i 15 C. In the measurement of specific electrical conductivity, Franck and Hensel found that the values thereof decrease 5 to 6 orders of magnitude just above the critical pressure when increasing the mercury temperature from room temperature to the critical temperature. However, Franck and Hensel also found that if the pressure is allowed to vary, so as to increase from 1,580 to 2,100 bars at 1,520 C the conductivity increase by four orders of magnitude. If one is to take advantage of the critical pressure in obtaining a current limiting effect, the pressure must be only allowed to vary within a narrow range that is between 1,510 i 25 bar and 1,580 bar. In the preferred embodiment of this invention, the pressure of the mercury is controlled at a value of approximately 1,535 bar while the temperature of the mercury in the capillary, which will be described hereafter, is raised at least to the critical temperature by the heating effect of electrical current flowing therein. By examining the curve of FIG. 1, it can be seen that the specific conductivity (rho,,rh0,,) or (6,,l6 or the ratio of the conductivity of mercury at any temperature to the conductivity of mercury at zero degrees centigrade is plotted on the vertical axis. The range varies upwardly from 10 to 1. On the horizontal axis pressure is plotted from 1,200 to 2,200 bars. The family of curves depicted in FIG. 1 are distinguished one from another by the parametric value of temperature ranging from 0 C to 1,700 C for mercury. At 0 C regardless of the pressure the specific conductivity is always 1. As the temperature is increased to 1,200 C the conductivity decreases to approximately 1/10 of the conductivity at 0 C but nevertheless remains relatively unvaried as a function of pressure. However, after 1,200 C conductivity begins to become a strong function of pressure. At 1,490 C, the critical temperature, the characteristic conductivity of mercury is strongly non-linear and exhibits a radical break point or knee at or approximate to the critical pressure of 1,510 bar. This strong non-linear characteristic is exhibited through approximately l,520 in the temperature range. After which the effect of temperature and pressure on the specific conductivity of mercury begin to show a more linear-like trend. Below the critical pressure of 1,510 i 25 bar large excursions in temperature from 0 C to any value up to less than 1,490 C will eventually lead to sublimation of the liquid mercury into a gaseous state as is typical of rehealable current limiting fuses of the mercury type. An excursion of temperature in this region will cause a change in conductivity of at the most of 3 orders of magnitude before the undesirable formation of gas takes place. The formation of gas has a current limiting characteristic but it also leads to the production of a detrimental electric arc. Extremely large excursions in temperature for pressures between 1510 i 25 bar and 1580 bar will lead to dramatic and sudden changes of specific conductivity (6 /6 approximately 6 orders of magnitude, without causing sublimation of the mercury liquid into a gas. This operating pressure region is designated 12 in 1. For pressures above 1,580 bars vary large excursions in temperatures up to approximately 1,700 in magnitude starting at room temperature or thereabout will produce dramatic changes in specific conductivity. However, as can be seen by closely examining the curves of FIG. 1, it would take the large excursion of temperature 1 ,700) to achieve the changes in specific conductivity required in this pressure range. By way of illustration, presume that the mercury is heated from 0 C to a temperature of slightly less than l,490 C at a pressure slightly less than the critical pressure. This is designated by the line 14 of FIG. 1. It will be noted that a decrease in specific conductivity will occur of approximately 3 orders of magnitude after which sublimation will occur and the potential for the formation of a detrimental arc will be high. Contrast this with the change of the temperature of the mercury from 0 C to l,490 C the critical temperature, at a pressure of 1510 i bar the critical pressure. This will cause a change in specific conductivity of approximately 6 orders of magnitude without causing the sublimation of the mercury to the gaseous state. Finally, presume the mercury is heated from 0 C to approximately l,490 C at a pressure slightly above 1580 bar. This will only cause the specific conductivity of the mercury to vary by 2 orders of magnitude although no sublimation will occur. From the foregoing, example which is illustrative and not limiting, it can easily be seen that relatively smaller excursions of temperature, approximately 1,500 within a critical range of pressure from about 11510 i 25 bars to 1,580 bars will produce dramatic changes in specific conductivity whereas the same excursions in temperature at lessor or greater pressures will not cause the same effect. The recognition of the characteristics of F I6. I in the narrow range through which desirable results all attained have led to the conclusion that mercury under controlled conditions can be used as a current limiting medium in a way which has not been heretofore used. Namely, the change of conductivity by large magnitude without the sublimation of the mercury.

Referring now to FIG. 2 there is shown a liquid state current limiter 20. Current limiter 20 is connected in circuit relationship with a load L to be protected by the current limiting action of the current limiter 20, a switch, circuit breaker or circuit interrupter B, and a source of energy or power supply F. During certain operating conditions an electrical current I which may be alternating flows in the circuit. The preceding elements form a current limiting circuit interrupter. At times it may be desirous to limit and/or stop the current I from flowing. Current limiter 20 comprises a high-pressure electrically conducting body 22 on the left which forms one terminal of the liquid stage current limiter 20. On the right there is another high-pressure electrically conducting body 24 which forms the other lead for the liquid stage current limiter 20 disposed between highpressure conducting bodies 22 and 24 is a locating and insulating body. 26. The three bodies 22, 24, and 26 are joined by bolts which in the preferred embodiment of the invention may be six in number which extend longitudinally between aligned openings or holes in all of the boies 22, 24, and 26. Bolts are fastened by way of nuts 28A to secure the three bodies 22, 24, and 26 together. In insulating body 22 an insulating cylinder 30 is provided in each of the previously described bolt holes for insulating the bolt 28 from the conducting body 22. In a like manner a similar insulating member 32 is provided adjacent one side of the conducting body 22 to form an end piece or end cover for the liquid stage current limiter. A fill hole 34 is provided in the highpressure conducting body 22. The fill hole 34 has a threaded portion therein to which a fill hole closing bolt 34A may be threaded to close off the fill hole 34. There is a central opening 348 which communicates with either side of the conducting body 24. There is a central opening 34C in the locating and insulating body 26 which communicates with either side of the locating and insulating body 26 and with the fill hole 34 and the central opening or hole 348 in the high-pressure conducting body 24. Disposed in hole 34C is a refractory insulator 36 which may comprise sintered alumina or sapphire. This heat resistive member 36 has a capillary central hole 38 which completes the communication between the fill hole 34 and the central opening or reservoir opening 348 of the conducting body 24. O-rings 35 are disposed in sealin g configuration in proper channels which are extensions of sill hole 34 and reservoir 34B respectively between the conducting body 22 and the locating and insulating body 26 and the conducting body 24 and the locating and insulating body 26, respectively. It should be noted that the O-rings 35 or similar sealing means need only provide sealing for the leakage of mercury under high-pressure from the fill hole 34 or the reservoir 34B and points external to the liquid stage current limiter 20. There is no need to pro vide gas insulation as it is not necessary and undesirable to sublime the mercury which is under pressure to the gaseous state. Disposed in a portion of the reservoir 34B is a siphon bellows or diaphragm 42 which is adapted to flex longitudinally or otherwise to apply pressure to fluid which may be constrained within the reservoir 348. The siphon-bellows or diaphragm 42 has disposed along one end thereof a gas chamber 44. The gas chamber 44 has a valve 45 disposed at an entrance port thereto which communicates with a multi-stage compressor 46 which in turn communicates with a supply tank 47. Disposed within the communicating openings 34, 38, and 34B is a pool of liquid mercury 48, a portion of which is disposed in the capillary 38. The mercury is provided to the previously mentioned openings in a convenient manner such as by back filling after evacuation through the filling hole 34, after which the filling hole closing bolt 34A is tightened down in a sealable manner creating a completely enclosed pool of mercury 38 in the filling hole 34, capillary 38 and the reservoir 34B. Pressure may then be applied through the gas chamber 44 to the bellows 42 causing it to flex to compress the mercury 38 to the desirable pressure which is in this case the critical pressure. The electrical current I flows through the conducting body 22 the mercury of the fill hole 34, the mercury in the capillary 38, the mercury in the reservoir 348, the conducting body 24, the load I. and the closed switch breaker or interrupter B. The current is supplied by the supply S which completes the circuit. The size of the capillary 38 and other parameters determine at what value of current I the mercury 48 will heat to a temperature about the critical temperature causing the mercury 38 in the capillary 48 to rapidly change specific conductivity by 6 orders of magnitude thus drastically limiting the current I to a very low value sufficient to allow the breaker or interrupter B to be opened at the next current zero or otherwise in a very convenient and safe manner thus completely interrupting the flow of current I. The small narrow capillary 38 allows the mercury 48 disposed therein to be changed in temperature easily to provide a high degree of sensitivity for the liquid stage current limiter. If the capillary were larger and included a large mass of mercury such as may be present in the sill hole 34 or reservoir 348 it would be more difficult to heat it to the desired temperature with the kind of response time necessary to cause current limiting as quickly as is usually desired in a commercial operating situation. It will be noted that at no time is the mercury 48 vaporized. Consequently no arcs are drawn in the current limiter 20. It can be seen that current I may be in the kiloamp range before current limiting occurs and may be reduced to the miliamp range after current limiting occurs. The breaker B therefore does not have to interrupt kiloamps of current but merely how to interrupt miliamps of current. The breaker B can be effective for interrupting high values of current even though the effective rating of the breaker may be minimal because of the current limiting action of the liquid stage current limiter 20.

In the preferred embodiment of the invention the high pressure conducting bodies are made of high strength stainless steel type 304. The same material may be used for the studs or bolts 28. The O-rings may be formed from nickel, iron or pure iron. In order for the mercury pressure to be held nearly constant, the ratio of the mercury volume to gas volume should be very small, 0.1 for example. The mercury volume must be able to increase as mercury temperature increases. This is done by the motion of the siphonbellows or diaphragm 42 to compress the helium, which is the gas used in the preferred embodiment of the invention in the gas chamber 44. The compressability of helium in the volume of the gas chamber are such as to reflect only a small increase in the equalizing pressure of the mercury and helium volumes when the mercury is raised to its critical temperature. The helium in the gas chamber 44 is ideally maintained at 1,535 bar to transmit that pressure to the mercury 48 in the capillary 38 to maintain the mercury at approximately 1,535 bar. The siphon-bellows 42 may be made of stainless steel. It will be noted that after the mercury heats to a current limiting status and the current is reduced either by the current limiting action of the current limiter or by the opening or the breaker B the mercury will once again cool to a conducting state so that current I may once again flow at a desirable value. It will be noted that the mercury 48 completely fills all the communicating orifices 34, 38 and 348 under pressure. Consequently it can easily be seen that the attitude and disposition of the current limiter 20 is relatively unimportant as the effect of gravity and spillage is practically nonexistant.

It is to be understood that mercury is not the only material that is invisioned as being useful in a liquid stage current limiter. Potassium and sodium among others may also be available as may other alkali type materials. The material which is used for current limiting gen erally may be described as a liquid electrically conducting material which has a critical temperature and temperature which critical temperature and pressure are attainable and are usable in laboratory and commercial situations and apparatus. As an example, some materials may have liquid electrically conducting properties but the attainment of either their critical pressure and- /or critical temperature may be too difficult for useful application in a current limiting mode. In other situations the liquid material or metal may have an attainable or useful critical temperature or pressure but these values may not be attainable by known apparatus. It should be understood however that the capabilities of known apparatus to constrain temperatures and pressures as well as the capabilities of the materials being used to achieve these temperatures and pressures are not limiting and as such critical temperatures and pressures are made useful by advances in the art their use in a mode for providing current limitation by varying specific conductivity over a wide range of orders of magnitude without changing state are covered by the inventive concept of this invention. It is also to be understood that the specific configuration of FIG. 2 is not limiting and other apparatus may be used for providing the pressure-temperature characteristics necessary to achieve current limitation as described. It is also to be understood that the multi-stage compressor 46 and supply tank 47 need not be connected at all times to the current limiter. Once the achievement of 1,5 35 bars of helium or any other acceptable pressure has been attained in the gas chamber 44 the use of the multi-stage compressor 46 and supply tank 47 is minimized. It is also to be understood that the current limiter may be used as a current raiser or current changing device by lowering the temperature or using the previously described apparatus in revision, i.e., lower the temperature to raise current.

The apparatus and techniques involving the teachings of this invention have many advantages. One of which is the fact that no steady energy application is necessary to render the liquid stage current limiter 20 operative. This is opposed to the situation of a cryogenic current limiter where large amounts of energy must be provided to continuously cool the current limiting device so that it may be warmed by the application of the overload current to cause current limitation. Another advantage lies in the fact that large sweeps in conductivity, up to 6 orders of magnitude, are achieveable with a mercury liquid stage current limiter. Another advantage lies in the fact that the sealing means 35 need not seal against gas leakage but need only seal against the leakage of liquid as it is not invisioned that a gas will be formed at any time during the operation of the current limiter. Another advantage lies in the fact that the current limiter can be made relatively small and portable. Another advantage lies in the fact that since no arc is produced or invisioned, the deteriorating effects of an electrical arc are not present in the current limiter and it is invisioned that the current limiter will provide its current limiting function almost indefinitely without breaking down or deteriorating. This leads to the fact that the current limiter may be used in deep space of deep sea applications or in remote locations such as desert transmission stations where a high degree of reliability and high mean time between failures is desirable.

What I claim as my invention is:

l. A current limiting device, comprising:

a. a closed container having spaced terminals adapted to be connected to an external source of power and a load for containing therein liquid electrically conducting material; and

b. liquid electrically conducting material disposed within said container in electrical contact with said spaced terminals, said liquid being pressurized to the predetermined critical pressure of said liquid for a period of time concurrent with the time said liquid is subjected to the predetermined critical temperatures of said liquid by a predetermined value of electrical current flowing in said liquid be tween said spaced terminals to cause the conductivity of said liquid to change abruptly to a relatively low value with out said liquid vaporizing to thereby cause said electrical current to be limited.

2. The combination as claimed in claim 1 wherein said liquid electrically conducting material comprises a metal.

3. The combination as claimed in claim 2 wherein said metal comprises mercury.

4. The combination as claimed in claim 11 wherein said critical pressure lies in a range of 1510 i 25 bar and said critical temperature lies in a range of 1490 i 15 C.

5. A current limiting circuit interrupter comprising:

a. a current interrupted having separable main contact,

b. a closed container having spaced terminals adapted to be connected in circuit relationship to an external source of power and a load and said separable main contacts of said circuit interrupter, said container containing therein liquid electrically conducting material; and

c. liquid electrically conducting material disposed within said container in electrical contact with said spaced terminals, said liquidbeing pressurized to the predetermined critical pressure of said liquid for a period of time concurrent with the time said liquid is subjected to the predetermined critical temperature of said liquid by a predetermined value of electrical current flowing in said liquid between said spaced terminals to cause the conductivity of said liquid to change abruptly to a relatively low value without said liquid vaporizing to thereby cause said electrical current to be limited.

6. The combination as claimed in claim 5 wherein said liquid electrically conducting material comprises a metal.

7. The combination as claimed in claim 6 wherein said metal comprises mercury.

0. The combination as claimed in claim 5 wherein said critical pressure lies in a range of 1510 i 25 bar and said critical temperature lies in a range of 1490 i 15 C.

9. A current changing device comprising contained liquid electrically conducting material pressurized at its critical pressure at the same time that the temperature thereof is changed through a range which includes the later materials critical temperature, said device adapted to conduct electrical current, said liquid remaining substantially unvaporized but changing conductivity to therefore substantially change the value of electrical current flowing therethrough.

l l l= 

1. A current limiting device, comprising: a. a closed container having spaced terminals adapted to be connected to an external source of power and a load for containing therein liquid electrically conducting material; and b. liquid electrically conducting material disposed within said container in electrical contact with said spaced terminals, said liquid being pressurized to the predetermined critical pressure of said liquid for a period of time concurrent with the time said liquid is subjected to the predetermined critical temperatures of said liquid by a predetermined value of electrical current flowing in said liquid between said spaced terminals to cause the conductivity of said liquid to change abruptly to a relatively low value with out said liquid vaporizing to thereby cause said electrical current to be limited.
 2. The combination as claimed in claim 1 wherein said liquid electrically conducting material comprises a metal.
 3. The combination as claimed in claim 2 wherein said metal comprises mercury.
 4. The combination as claimed in claim 1 wherein said critical pressure lies in a range of 1510 + or - 25 bar and said critical temperature lies in a range of 1490 + or - 15* C.
 5. A current limiting circuit interrupter comprising: a. a current interrupted having separable main contact, b. a closed container having spaced terminals adapted to be connected in circuit relationship to an external source of power and a load and said separable main contacts of said circuit interrupter, said container containing therein liquid electrically conducting material; and c. liquid electrically conducting material disposed within said container in electrical contact with said spaced terminals, said liquid being pressurized to the predetermined critical pressure of said liquid for a period of time concurrent with the time said liquid is subjected to the predetermined critical temperature of said liquid by a predetermined value of electrical current flowing in said liquid between said spaced terminals to cause the conductivity of said liquid to change abruptly to a relatively low value without said liquid vaporizing to thereby cause said electrical current to be limited.
 6. The combination as claimed in claim 5 wherein said liquid electrically conducting material comprises a metal.
 7. The combination as claimed in claim 6 wherein said metal comprises mercury.
 8. The combination as claimed in claim 5 wherein said critical pressure lies in a range of 1510 + or - 25 bar and said critical temperature lies in a range of 1490 + or - 15* C.
 9. A current changing device comprising contained liquid electrically conducting material pressurized at its critical pressure at the same time that the temperature thereof is changed through a range which includes the later material''s critical temperature, said device adapted to conduct electrical current, said liquid remaining substantially unvaporized but changing conductivity to therefore substantially change the value of electrical current flowing therethrough. 